1. Field of the Invention
This invention relates to a process for the production of fatty acid polyethylene glycol esters by ethoxylation of fatty acids in the presence of alkanolamines.
2. Statement of the Related Art
The addition of alkylene oxides to CH-acidic compounds such as, for example, fatty alcohols, alkylphenols, fatty amines or even fatty acids is among the industrially established processes for the production of nonionic surfactants. These reactions are normally carried out in the presence of homogeneous basic catalysts such as, for example, sodium hydroxide or sodium methylate. However, alkoxylation is not a selective reaction, but one which follows statistics. Accordingly, the average degree of alkoxylation can at best be estimated from the molecular ratio in which the CH-acidic compound and the alkylene oxide are used. In practice, however, the maximum of the resulting homolog distribution--particularly at low alkoxylation ratios--is never to be found in the average degree of alkoxylation, but is generally displaced towards higher homologs.
Attempts have been made to counteract this unwanted effect by using catalysts with a higher selectivity which, overall, lead to alkoxylates, particularly ethoxylates, having a narrower ("restricted") homolog distribution. These products are often referred to in the literature as narrow-range ethoxylates. Preferred homogenous catalysts for this purpose are alkaline earth metal salts such as, for example, barium phosphate or strontium ether carboxylates. Heterogeneous catalysts such as, for example, calcined hydrotalcites are also suitable.
However, the efforts which have hitherto been made in regard to the ethoxylation of fatty acids are still unsatisfactory. Above all in the production of fatty acids with low degrees of ethoxylation, particularly fatty acid +1 EO adducts, which are of interest as precursors for the synthesis of ether sulfate surfactants with an isethionate-like structure, the selectivities obtained are unsatisfactory. Apart from an unwanted content of homologs with a relatively high degree of ethoxylation, significant quantities of polyethylene glycol and diesters are also formed.
Accordingly, the problem addressed by the present invention was to provide an improved homogeneous chemical process for the production of fatty acid polyglyol esters, particularly fatty acids with low degrees of ethoxylation, which would be distinguished by improved selectivity.